I have a multiboot system set up. The system has three drives. Multiboot is configured with Windows XP, Windows 7, and Ubuntu - all on the first drive. I had a lot of unpartitioned space left on the drive and was reserving it for adding other OSes and for storing files there in the future.

One day I went ahead and downloaded Partition Wizard and created a logical NTFS partition from within Windows 7, still some unpartitioned space left over. Everything worked fine, until I rebooted the computer a few days later.

Now I'm getting:
error: unknown filesystem.
grub rescue>

First of all I was surprised not to find any kind of help command, by trying:
help, ?, man, --help, -h, bash, cmd, etc.

Now I'm stuck with non-bootable system. I have started researching the issue and finding that people usually recommend to boot to a Live CD and fix the issue from there. Is there a way to fix this issue from within grub rescue without the need for Live CD?

So far from reading the manual on grub rescue, I was able to see my drives and partitions using ls command. For the first hard drive I see the following:
(hd0) (hd0,msdos6) (hd0,msdos5) (hd0,msdos2) (hd0,msdos1)

I now know that (hd0,msdos6) contains Linux on it, since ls (hd0,msdos6)/ lists directories. Others will give "error: unknown filesystem."

UPDATE 2

After the following commands I am now getting to the boot menu and can boot into Windows 7 and Ubuntu, but upon reboot I have to repeat these steps.

Thanks Shashank Singh, with your instructions I have simplified my steps to the following. I have learned from you that I can replace msdos6 with just a 6 and that I can just do insmod normal instead of insmod /boot/grub/linux.mod. Now I just need to figure out how to save this settings from within grub itself, without booting into any OS.

set root=(hd0,6)
set prefix=(hd0,6)/boot/grub
insmod normal
normal

UPDATE 4

Well, it seems like it is a requirement to boot into Linux. After booting into Ubuntu I have performed the following steps described in the manual:

sudo update-grub
sudo grub-install /dev/sda

This did not resolve the issue. I still get the grub rescue prompt. What do I need to do to permanently fix it?

I have also learned that drive numbers as in hd0 need to be translated to drive letters as in /dev/sda for some commands. hd1 would be sdb, hd2 would be sdc, and so on. Partitions listed in grub as (hd0,msdos6) would be translated to /dev/sda6.

There is an alternative cause of this problem. In this particular case, grub was somehow corrupted and needed to be repaired or reinstalled. However, as shown on this thread, it's also possible that the root partition on which grub is installed could be corrupted. To fix this:

This is a possible solution, but it should not be used likely lest your root partition become further corrupted. Running the command: fsck -t ext4 /dev/sda1 This program attempts to search and repair errors on a corrupted filesystem. Replaced sda1 with your actual root partition. Replace ext4 with the actual filesystem, you have to know the file system or else the partition will be more corrupted. See this thread for more information.

Even though this question has an answer, there is an alternative way to fix the problem that worked for me. The steps are explained in this painful video: http://www.youtube.com/watch?v=ZcbTgMKpVHQ . In short, it will reinstall Grub2 altogether instead of repairing it.

Because this video is so painful to watch, I'll list the steps below (as I should regardless of how painful it is to watch the video)

Launch a live session of Ubuntu. The video uses a live CD where as I used a live-usb. I made sure that the live-usb has the same version of Ubuntu that I had on my harddrive.

Find where your root partition was mounted. In the video, the user uses nautilus to navigate through each drive that was mounted. It was mounted with a long string of numbers and characters. If this is the case, follow the following steps to remount the partition. Else, procede to step 5.

Bring up the terminal with ctrl+alt+T and use the mount command to find the name of the partition.

Mount the partition. Create a new folder in your media folder. sudo mkdir /media/ubuntu. Then simple mount your partition to that folder. sudo mount /dev/sdxx /media/ubuntu where xx of sdxx is determined in step 3.

Bind the following directories from the root directory of your live cd/usb to that of your root directory on your version of Ubuntu. The directories are the devproc and sys. Do so with the following commands:

1. set prefix=(hdX,Y)/boot/grub
Use the values determined earlier. Example: If the Ubuntu system is on sda5, enter: set prefix=(hd0,5)/boot/grub
2.* set root=(hdX,Y)
Example: set root=(hd0,5)
3. insmod normal
Attempt to load the normal module.
4. normal

I have never tried a grub rescue, but since I have read about it, just curious.
what did you get when you tried set root=(hd0,6)/boot/grub and the following insmod normal?

We basically need grub to know where the /boot/grub folder is. If this is not working, I guess doing a grub install from a boot cd is a good option

Glad it worked for you. I came to know about the Boot-repair disk from this. May come handy. However, the grub-install should have worked. Did the terminal say anything particular after that command?

Thanks for your assistance. I think my problem may have been that my boot directory is on /dev/sdb, not /dev/sda. But boot-repair fixed it automatically, so I did not have to do anything further.
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user53817Apr 15 '12 at 2:52

Thanks for your answer, but I got my grub fixed using boot-repair tool before you posted the answer. I have now updated this post with my solution. I wish I could have tried your method as well.
–
user53817Apr 7 '12 at 8:21